There has been conventionally known an automatic transfer apparatus equipped with a self-propelled transfer vehicle such as a tractor type self-propelled vehicle (hereinafter called a carriage tractor) that is attachable and detachable through a connecting portion to a carriage loaded with workpieces or loads and runs by self-propulsion along a predetermined path by using guiding means such as magnetic information laid on a floor surface while towing the carriage, or a carrier type self-propelled vehicle that is integrated through a connecting portion with a carriage loaded with workpieces or loads and runs by self-propulsion along the predetermined path by using the guiding means such as the magnetic information laid on the floor surface.
Now, as shown in FIG. 12, a carriage tractor 50 employed in the conventional automatic transfer apparatus is always integrally connected with a carriage 4 near the substantial center thereof. In addition, the carriage tractor 50 is arranged with two units (2WD+2WS) of drive wheel units 7, 7 having a pair of independently rotating right and left wheels 8, 8, in a manner separated from each other in the front-rear direction. Note that idler wheels 25a to 25d are respectively provided at four corners of the bottom of the carriage 4.
In recent years, effective use of the automatic transfer apparatus tends to increase. Moreover, it is sought to downsize the carriage tractor so as to enable running in a small space for higher efficiency, and it is also sought to achieve lower cost.
However, in order to configure the carriage tractor (2WD+2WS) 50 having the complicated structure as shown in FIG. 12 so as to be capable of running in the traverse direction converted from the forward-reverse direction, each of the two drive wheel units 7, 7 needs to be additionally provided with such a device as a steering control motor or a steering angle sensor to convert the direction of the drive wheel units 7, 7 from the forward-reverse direction to the traverse direction as shown in FIG. 12B. Therefore, the structure becomes further complicated, disabling the downsizing. In addition, in the case in which the carriage tractor 50 is configured to have one drive wheel unit 7 for downsizing the carriage tractor 50, when the carriage tractor 50 runs in the traverse direction while towing the carriage 4, the carriage 4 is laterally displaced with respect to the running direction to result in running in an oblique direction at the time when the idler wheels 25a to 25d of the carriage 4 convert the direction thereof from the forward-reverse direction to the traverse direction. Therefore, in order to restrict the lateral displacement of the carriage 4 with respect to the running direction thereof, the carriage tractor 50 requires enough driving force to overcome the force displacing the carriage 4. Consequently, it is required to make the drive wheel unit 7 extremely large or to increase the number thereof to at least two units or more, thereby making difficult to downsize the carriage tractor 50.
It should be noted that, as a related art of the automatic transfer apparatus, Patent Document 1 describes a four-wheel steering type unmanned transfer vehicle that is an unmanned transfer vehicle switchable between travel and traverse. The unmanned transfer vehicle is provided, on the lower surface thereof, with four running wheels having steering means for making steer angles of the wheels controllable independently of each other, and two of the running wheels arranged on the diagonal line of the transfer vehicle are provided with running drive motors so as to enable travel and traverse, thereby maintaining equilibrium of driving force with respect to the transfer vehicle and obtaining running stability.    Patent Document 1: Japanese Patent Application Publication No. JP-A-8-123550